Sharing Calendars Across Platforms
Keep your team on schedule, even if they're not on the same operating system
Clients Able to Access the Standards
Since iCalendar is a subset of CalDAV, the issues surrounding support for either of these standards can blur together. Remember that iCalendar is a file format and CalDAV is the Web-sharing component. The list of software that supports iCalendar is extensive, including Google Calendar, Apple iCal, the iPod, Lotus Notes with Domino version 6 or later, KOrganizer, Mozilla Calendar/Mozilla Sunbird, Novell Evolution, Windows Calendar (included with Microsoft's upcoming Vista release), and Microsoft Outlook (to some extent, depending on its version). Google Calendar and Mozilla Calendar/Sunbird are both cross-platform solutions.
For example, if your users are running exclusively Microsoft Outlook 2000, 2002, or 2003, they cannot interact properly using iCalendar. However, there is a known problem with sending iCalendar meeting requests from Outlook 2000 and 2002 to Outlook 2003 users under certain situations (see the link for more details). Outlook 2000 users are also known to have problems opening iCalendar files sent to them that were created by later versions of Outlook. According to the Microsoft Knowledge Base, the Outlook 2000 Post-Service Pack 3 Hotfix Package July 3, 2003 contains a fix for this problem. This trouble also appears when sending iCalendar data from other clients (such as Novell Evolution) to Outlook versions 2000 and 2002.
Since Outlook typically uses Microsoft protocols for publishing calendar information, you would also need to have your users override this feature so they can use iCalendar. While the screenshots would look slightly different for each version of Outlook, the actual steps are the same as they have been since Outlook 2000. For versions of Outlook earlier than Outlook 2007, actually subscribing to an iCalendar requires workarounds.
According to Microsoft, Outlook 2007 will fully support the iCalendar and CalDAV standards. In the meantime, Windows users can choose Mozilla Sunbird, Microsoft Works 8 or 2006, EventSherpa, or Google Calendar to interact with iCalendar and CalDAV online. There is also a plugin called Remote Calendars for Outlook 2003, though many recommend testing this plugin to be sure that it works as you need it to before giving it to your users.
iPod users cannot subscribe to CalDAV data, but they can receive iCalendar data, so the recommended workaround is to subscribe using other software and then export the calendars into iCalendar format to synchronize onto the iPod. Lotus Notes users interacting with Domino version 5 and earlier will receive iCalendar data as an .ics attachment which they can save and use in other software. However, Lotus Notes does not support subscribing to calendars through WebDAV.
Servers for Cross-Platform Solutions
When it comes to collaboration servers that can support iCalendar and WebDAV/CalDAV sharing across clients on multiple platforms, there aren't quite as many choices. Microsoft Exchange itself does support WebDAV, and clients such as Novell Evolution for Linux do have "connectors" available that can talk to Microsoft Exchange servers. The combination of Microsoft Exchange and Microsoft Outlook for the Macintosh, however, does not always support iCalendar. There are also some known problems with various versions of Exchange, such as two with iCalendar meeting requests passing through Exchange 2000: see "XCON: Meeting Requests That Use the iCalendar Format Are Not Processed Correctly" and "An iCalendar meeting request or response becomes stuck in the local delivery queue."
The major choices for Microsoft Exchange replacements are Zimbra Collaboration Suite, PostPath, and Scalix. All of these solutions also offer a Web-based client along with regular client support including Outlook, giving you multiple options for how to handle your cross-platform needs. However, Scalix does not, at the time of this writing, support iCalendar (and, hence, CalDAV). Apparently there is a feature request under consideration, so this feature may be supported in the future. Zimbra as a server is available for both Linux and Macintosh OS X. The Zimbra Web client has also been tested thoroughly under Microsoft Windows XP.
PostPath is exclusively a Linux-based server. However, its Web client is also cross-platform and cross-browser.
As you can see, calendar sharing is still a bit of a mess when trying to cross platforms or even cross different product families on the same platform. However, over the years convergence toward iCalendar and CalDAV has grown across the industry. With the release of Microsoft Vista and Microsoft Office 2007/12, this convergence will take a large step forward.